This invention relates to non-invasive, small-perturbation measurements of macroscopic mechanical properties of organs and blood vessels to evaluate tissue pathology and body function.
Pathological changes in tissues are often correlated with changes in the mechanical properties of density, elasticity and damping. While microscopic mechanical changes have sometimes been correlated with ultrasound wavelengths and frequencies, many important mechanical changes are manifested most clearly on a large scale at low frequencies down to zero. For these, manual palpation remains almost the sole diagnostic tool. A great deal of effort has been expended in the area of blood pressure measurement, but not by analyzing small-perturbation mechanical properties of the pressurized vessel.
Arterial blood pressure measurement methods are commonly either invasive (catheterization or cannulation) or else disruptive mechanical perturbations, typically causing temporary occlusion of blood flow, e.g. by a sphygmomanometer cuff. Pulmonary arterial pressure is so inaccessible that it is seldom measured. The trauma of entering any artery is an obvious disadvantage. Most occlusive methods are only capable of sampling the systolic and diastolic extremes of the blood pressure waveform. Occlusive methods cannot be used for extended monitoring because of the interruption of circulation.
Recent less occlusive pressure monitoring methods include those described by Aaslid and Brubkak, Circulation, Vol. 4, No. 4 (ultrasound doppler monitors brachial artery flow while a servoed cuff maintains fixed, reduced flow) and Yamakoshi et al, "Indirect Measurement of Instantaneous Arterial Blood Pressure in the Human Finger by the Vascular Unloading Technique", IEEE Trans. on Biomedical Eng., Vol. BME-27, No. 3, March 1980 (a similar system optically monitors capillary blood volume in the finger while a servoed cuff maintains a constant optical reading).
Non-invasive blood pressure monitoring approaches suggested in prior art are described by Jeff Raines, Diagnosis and Analysis of Arteriosclerosis in the Lower Limbs, Ph.D. Thesis, M.I.T., Sept. 1972 (using a low-pressure cuff surrounding a limb to monitor the changing cross-section as enclosed arteries pulsate in diameter) and by D. K. Shelton and R. M. Olson, "A Nondestructive Technique To Measure Pulmonary Artery Diameter And Its Pulsatile Variations", J. Appl. Physiol., Vol. 33, No. 4, Oct. 1972 (using an ultrasound transducer in the esophagus to track canine pulmonary artery diameter). The latter investigators reported approximate short-term pressure/diameter correlation, while Itzchak et al, "Relationship of Pressure and Flow to Arterial Diameter", Investigative Radiology, May-June, 1982, using ultrasound to track canine arterial diameter, found no useful longterm pressure/diameter correlation.
In other areas of the human body, Kahn, U.S. Pat. No. 3,598,111, describes a mechanically and acoustically tuned pneumatic system, useful at a single frequency, for measuring the impedance of the air passages and tissues of human lungs to obtain a two-component trace (representing resistive and reactive impedance) as a function of time.